The 5G communication systems target on ultra-high data rate over several Giga bits per second. Communication between cellular base-station (BS) and indoor users (IDU) is one of the most common situation in daily life and establishing a high-speed link in this scenario is critical for deployment of 5G systems. Advanced techniques have been proposed to achieve the promised high data rate, such as millimeter wave (mmWave), beamforming, MIMO, etc. Although these techniques may possess potential for high data rate in certain scenarios, they often face challenges in this outdoor-to-indoor access situation.
MmWave is considered as one of key techniques for 5G as its broadband spectrum resource enables high speed data communication. However, the high propagation loss as described by Friis Transmission equation, limits the coverage range of single mmWave BS. Beamforming may be used to extend the range coverage with the trade-off between the high directivity and the angular coverage. In existing techniques, mmWave relay system is proposed to extend the coverage range. However, despite of the propagation loss, the signal strength drops dramatically when mmWave penetrates through obstacles. The outdoor to indoor communication is one of such non-line-of-sight (NLOS) scenarios. Prior researches show that the penetration loss of 28 GHz and 38 GHz signal through common building materials vary from 20 to 40 dB. One may consider using wired connections based on optic fibers. But deployment of optic fibers into existed buildings could be difficult and expensive and thus a wireless link is necessary. On the other hand, direct relay the mmWave to lower frequency bands over relays installed on the windows have been proposed by AT&T solution. The limited bandwidth, however, in the lower frequency bands creates bottleneck of information flow and it cannot support the relay to high speed data links in mmWave.
U.S. Pat. No. 9,294,162 to Nilsson et al. (“the '162 patent”) discloses a wireless outdoor-indoor multiple-input multiple-output (MIMO) communications system for communicating with user equipment located inside a physical structure such as a building. The MIMO communication system is comprised of a node having at least two node antennas, wherein the node is configured for line of sight (LOS) wireless MIMO communication with at least two outdoor-indoor repeaters, and of at least two outdoor-indoor repeaters adapted for LOS wireless MIMO communication with the node. The repeaters have at least one repeater antenna each, provided outside the physical structure, for LOS MIMO communication with the node and at least one leaky cable each, provided inside the physical structure, for indoor MIMO communication with the user equipment located inside the physical structure. However, the '162 patent does not teach or suggest using frequency-translational relaying units (RU) to link the broadband mmWave link outside the buildings and the microwave band MIMO link penetrating from outdoor to indoor.
Other researchers disclose that outdoor MIMO and indoor mm Wave point to point communication are united to form the hybrid relay system. Inside a building, frequency multiplexing at mmWave offers broad bandwidth between users and the relay unit. The advantages of low penetration loss at UHF band is utilized to transmit signal from outdoor BS to indoor Relay Unit (RU). As the narrow bandwidth at UHF band cannot match the broad bandwidth at mmWave, MIMO at UHF band is proposed to improve the channel capacity. However, spatial diversity from cellular base-stations to indoor users is usually limited at UHF due to its small angular spread and it conflicts with techniques such as beamforming or SDMA and negatively impact spectrum reutilization.